193 lines
7.3 KiB
ReStructuredText
193 lines
7.3 KiB
ReStructuredText
:mod:`functools` --- Higher-order functions and operations on callable objects
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==============================================================================
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.. module:: functools
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:synopsis: Higher-order functions and operations on callable objects.
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.. moduleauthor:: Peter Harris <scav@blueyonder.co.uk>
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.. moduleauthor:: Raymond Hettinger <python@rcn.com>
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.. moduleauthor:: Nick Coghlan <ncoghlan@gmail.com>
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.. sectionauthor:: Peter Harris <scav@blueyonder.co.uk>
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.. versionadded:: 2.5
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**Source code:** :source:`Lib/functools.py`
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--------------
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The :mod:`functools` module is for higher-order functions: functions that act on
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or return other functions. In general, any callable object can be treated as a
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function for the purposes of this module.
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The :mod:`functools` module defines the following functions:
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.. function:: cmp_to_key(func)
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Transform an old-style comparison function to a :term:`key function`. Used
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with tools that accept key functions (such as :func:`sorted`, :func:`min`,
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:func:`max`, :func:`heapq.nlargest`, :func:`heapq.nsmallest`,
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:func:`itertools.groupby`). This function is primarily used as a transition
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tool for programs being converted to Python 3 where comparison functions are
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no longer supported.
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A comparison function is any callable that accept two arguments, compares them,
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and returns a negative number for less-than, zero for equality, or a positive
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number for greater-than. A key function is a callable that accepts one
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argument and returns another value to be used as the sort key.
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Example::
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sorted(iterable, key=cmp_to_key(locale.strcoll)) # locale-aware sort order
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For sorting examples and a brief sorting tutorial, see :ref:`sortinghowto`.
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.. versionadded:: 2.7
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.. function:: total_ordering(cls)
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Given a class defining one or more rich comparison ordering methods, this
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class decorator supplies the rest. This simplifies the effort involved
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in specifying all of the possible rich comparison operations:
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The class must define one of :meth:`__lt__`, :meth:`__le__`,
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:meth:`__gt__`, or :meth:`__ge__`.
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In addition, the class should supply an :meth:`__eq__` method.
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For example::
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@total_ordering
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class Student:
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def __eq__(self, other):
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return ((self.lastname.lower(), self.firstname.lower()) ==
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(other.lastname.lower(), other.firstname.lower()))
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def __lt__(self, other):
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return ((self.lastname.lower(), self.firstname.lower()) <
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(other.lastname.lower(), other.firstname.lower()))
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.. versionadded:: 2.7
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.. function:: reduce(function, iterable[, initializer])
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This is the same function as :func:`reduce`. It is made available in this module
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to allow writing code more forward-compatible with Python 3.
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.. versionadded:: 2.6
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.. function:: partial(func[,*args][, **keywords])
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Return a new :ref:`partial object<partial-objects>` which when called will behave
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like *func* called with the positional arguments *args* and keyword arguments *keywords*.
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If more arguments are supplied to the call, they are appended to *args*. If
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additional keyword arguments are supplied, they extend and override *keywords*.
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Roughly equivalent to::
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def partial(func, *args, **keywords):
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def newfunc(*fargs, **fkeywords):
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newkeywords = keywords.copy()
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newkeywords.update(fkeywords)
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return func(*(args + fargs), **newkeywords)
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newfunc.func = func
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newfunc.args = args
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newfunc.keywords = keywords
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return newfunc
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The :func:`partial` is used for partial function application which "freezes"
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some portion of a function's arguments and/or keywords resulting in a new object
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with a simplified signature. For example, :func:`partial` can be used to create
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a callable that behaves like the :func:`int` function where the *base* argument
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defaults to two:
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>>> from functools import partial
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>>> basetwo = partial(int, base=2)
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>>> basetwo.__doc__ = 'Convert base 2 string to an int.'
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>>> basetwo('10010')
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18
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.. function:: update_wrapper(wrapper, wrapped[, assigned][, updated])
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Update a *wrapper* function to look like the *wrapped* function. The optional
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arguments are tuples to specify which attributes of the original function are
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assigned directly to the matching attributes on the wrapper function and which
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attributes of the wrapper function are updated with the corresponding attributes
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from the original function. The default values for these arguments are the
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module level constants *WRAPPER_ASSIGNMENTS* (which assigns to the wrapper
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function's *__name__*, *__module__* and *__doc__*, the documentation string) and
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*WRAPPER_UPDATES* (which updates the wrapper function's *__dict__*, i.e. the
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instance dictionary).
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The main intended use for this function is in :term:`decorator` functions which
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wrap the decorated function and return the wrapper. If the wrapper function is
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not updated, the metadata of the returned function will reflect the wrapper
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definition rather than the original function definition, which is typically less
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than helpful.
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.. function:: wraps(wrapped[, assigned][, updated])
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This is a convenience function for invoking :func:`update_wrapper` as a
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function decorator when defining a wrapper function. It is equivalent to
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``partial(update_wrapper, wrapped=wrapped, assigned=assigned, updated=updated)``.
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For example::
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>>> from functools import wraps
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>>> def my_decorator(f):
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... @wraps(f)
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... def wrapper(*args, **kwds):
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... print 'Calling decorated function'
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... return f(*args, **kwds)
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... return wrapper
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...
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>>> @my_decorator
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... def example():
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... """Docstring"""
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... print 'Called example function'
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...
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>>> example()
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Calling decorated function
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Called example function
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>>> example.__name__
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'example'
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>>> example.__doc__
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'Docstring'
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Without the use of this decorator factory, the name of the example function
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would have been ``'wrapper'``, and the docstring of the original :func:`example`
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would have been lost.
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.. _partial-objects:
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:class:`partial` Objects
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------------------------
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:class:`partial` objects are callable objects created by :func:`partial`. They
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have three read-only attributes:
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.. attribute:: partial.func
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A callable object or function. Calls to the :class:`partial` object will be
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forwarded to :attr:`func` with new arguments and keywords.
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.. attribute:: partial.args
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The leftmost positional arguments that will be prepended to the positional
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arguments provided to a :class:`partial` object call.
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.. attribute:: partial.keywords
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The keyword arguments that will be supplied when the :class:`partial` object is
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called.
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:class:`partial` objects are like :class:`function` objects in that they are
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callable, weak referencable, and can have attributes. There are some important
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differences. For instance, the :attr:`~definition.__name__` and :attr:`__doc__` attributes
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are not created automatically. Also, :class:`partial` objects defined in
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classes behave like static methods and do not transform into bound methods
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during instance attribute look-up.
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